The articles a and an are singular. Use a or an with singular nouns.
I saw an ocean.
I rode a bike.
The article the can be used with singular nouns if you are talking about a specific singular noun.
I saw the ocean.
I rode the bike.
Plural nouns must use the article the.
The is the only article that can be used with plural nouns.
I saw the two dogs
I found the books on the windowsill.
Remember: You can use the articles a, an, and the with singular nouns. With plural nouns, you can only use the article the.